HR: 1340h
AN: PP43B-1278    [Abstracts]
TI: Authigenic uranium in the sediments in the La Paz Bay and La Paz Basin, south-western Gulf of California
AU: * Choumiline, K
EM: constan_ayanami@hotmail.com
AF: Universidad Autónoma de Baja California Sur, Carretera al sur s/n. Col. El Calandrio, La Paz, BCS 23090, Mexico
AU: Herguera, J C
EM: herguera@cicese.mx
AF: Centro de Investigacion Cientifica y de Educacion Superior de Ensenada, Km. 107 Carretera Tijuana-Ensenada, Ensenada, BCS 22830, Mexico
AU: Shumilin, E
EM: eshumili@ipn.mx
AF: Centro Interdisciplinario de Ciencias Marinas - Instituto Politecnico Nacional, Avenida de IPN s/n, Col. Playa Palo de Santa Rita, La Paz, BCS 23096, Mexico
AU: Sapozhnikov, D
EM: sapozz@yandex.ru
AF: V.I.Vernadsky Institute of Geochemistry and Analytical Chemistry, Kosygin Str #19, Moscow, 119991, Russian Federation
AB: The presence of high concentrations of uranium in sediments is under research in order to explain this phenomenon, because of the possibility to use it as a proxy for reconstruction of past redox conditions in the marine environment. To characterize U sedimentary behavior in the south-western Gulf of California, four sediment cores were collected in the La Paz Lagoon, La Paz Bay and La Paz Basin. Core sediment subsamples were dried, homogenized and the contents of uranium and scandium were determined by an instrumental neutron activation analysis method. The average concentrations of the total U varied from 3.9±1.5 mg kg^{- 1} (core from the shallow 8 m-deep La Paz Lagoon) to 12.0±7.2 mg kg^-1 (core from the 400 m-deep entrance just between La Paz Bay and the adjacent open Gulf of California), 12.9±2.6 mg kg^-1 (core from the 400 m-deep suboxic Alfonso Basin of the La Paz Bay) and 13.3±2.6 mg kg^-1 (core from the 745 m–deep La Paz Basin, the Gulf of California). These concentrations are not the highest compared with other studies for the areas of coastal upwelling, but are still higher than uranium average abundance in Earth´s crust (2.7 mg kg^-1). The authigenic (non-lithogenic) uranium concentrations were calculated, using U/Sc crustal ratios, because Sc is a reliable indicator of terrigenous material, mainly aluminosilicates. The contribution of authigenic uranium were 46.7±21.3 % for the sediments of the La Paz Lagoon, 81.7±15.0 % for a core from the entrance at the limits of the La Paz Bay, 89.1±2.3 % for the sediments from the Alfonso Basin of the La Paz Bay and 90.6±2.2 for the sediment core from the La Paz Basin. The vertical profiles of this element in studied cores reveal different patterns of the variability of their contents in the sedimentary layers. Due to the sensibility of uranium to reducing conditions, authigenic uranium enrichments could reflect the occurrence of anoxic conditions in the water column in the past. Deeper waters in the study area are distinguished by the presence of oxygen minimum zone, which favours the less soluble uranium (IV) to precipitate on particles, whereas the dissolved uranium (VI) prevails in oxygenated upper layer of sea waters. The cores from the Alfonso Basin and La Paz Basin show some similarity and the variations of their U contents oscillate near mean value, indicating quiet conditions of the sediment accumulation. However, the core from the entrance of the La Paz Bay displays a clear change in the vertical distribution of the total U content at the 18 cm of core depth. In this core the total U content was in average 6.3±4.4 mg kg^-1 for the layer of sediments from 0 to 18 cm, and then it sharply increased to 16.3±5.8 mg kg^-1 for the layer from 18 to 40 cm. This pattern change could be a result of an underwater mass flow event, probably caused by tectonic seismic activity. This moved a layer of shallow sediments, depleted in authigenic uranium, to deeper places of the sea floor, characterized by higher authigenic uranium precipitation on it.
DE: 1065 Major and trace element geochemistry
DE: 3022 Marine sediments: processes and transport
DE: 4251 Marine pollution (0345, 0478)
DE: 4875 Trace elements (0489)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting